Optical diagnostics of flow velocity in pressure-matched near-vacuum gaseous micro jets

Near-vacuum gas flow is commonly encountered in vacuum equipment and high-end manufacturing processes such as extreme ultraviolet (EUV) light sources. Understanding and controlling the characteristics of the near-vacuum flow is crucial to optimizing the performance of these applications. In this study, we investigate the flow field of gas jets in low-pressure environments under varying gas compositions, flow rates, and vacuum chamber pressures using infrared molecular tagging velocimetry (IR MTV). This method eliminates the need to introduce tracer particles into the gas flow, thereby avoiding potential systematic errors in velocity measurements caused by reduced particle followability in low-pressure conditions.High spatially-resolved (29.4 $\upmu$m)velocity measurements were performed in the downstream region, 0.44$D$ away from the nozzle ($D$ is the diameter of the nozzle), with pressure controlledbetween 0.03 and 1 atm. The results show that the radial distribution of the jet velocity is independent of gas composition and flow rate and exhibits excellent similarity under the same pressure, while the maximum centerline velocity of the jet is proportional to the total gas flow rate. As the pressure decreases from 1 to0.03 atm,the op-hat structure of the jet flow field gradually disappears, and the radial attenuation ofaxial velocity intensifies. In addition, the pressure reduction causes the jet centerlinevelocity to exceed its average exit velocity. These findings demonstrate thattracer-particle-free infrared molecular tagging velocimetry is a suitable technique for near-vacuum gas flow measurementswith pressure less than 0.05 atm,providing high spatial resolution and accurate velocity distributions for both fundamental research and practical applications.